Inductive power transfer (IPT) without a magnetic core was first proposed by Nikola Tesla to supply wireless mains power over long distances around 100 years ago [1]. Since then, low-power, closely-coupled wireless charging methods have been used to power medical implants [2], while the wireless powering of portable devices through charging mats is now available via commercial products [3]. Nonetheless, there has been recent interest in wireless power transfer (WPT) for medium range (i.e. 10 s of cm) applications, such as electric vehicle charging through resonant inductive coupling [4]-[7].
For many industrial and commercial applications, IPT systems must be capable of achieving a high end-to-end efficiency gee, while transferring hundreds of watts at submeter distances, otherwise they will not be adopted. Several approaches for achieving good link efficiencies have been developed by several research groups. The first is to work at relatively low frequencies (tens of kHz), where efficient driver circuits can be easily realised and by increasing the coupling factor k of the system, using field-shaping techniques; for example, by employing metamaterials [8] and ferrite cores [6]. In [6], 2 kW of power was transferred at a distance of 10 cm using Litz wire coils at 20 kHz. The operating frequency was defined by the power handling capabilities of the coil driver, limiting the maximum coil unloaded Q-factor to 290. Field-shaping techniques normally occupy useful volume, require heavy materials, employ expensive fabrication techniques and need a precise coil alignment. These solutions make the field-shaping approach unsuitable for many applications, where the size, weight and cost of the system are limiting factors.
The second approach relies on transferring energy at the optimum frequency for maximum power transfer given a particular coil size, where the unloaded Q is maximised and compensates for the low coupling factor. In the past, this approach was not considered efficient, since low driver efficiency (due to semiconductor losses) dramatically reduced the end-to-end efficiency of the IPT system. An example of this was described by Kurs et al. [9], where the use of a 9.9 MHz Colpitts oscillator driver achieved an end-to-end efficiency of only 15%, when the transfer efficiency was 50%.
Other attempts at this approach have been successful, with the use of commercially-off-the-shelf (COTS) equipment to drive and impedance match the TX coils at frequencies above 3 MHz and with ηtransfer=95%, while also reducing the coil losses by using a surface spiral [10].
The highest ηee have been demonstrated by the commercial IPT systems currently available on the market. High efficiencies of ηee=90% have been achieved at distances of less than 30 cm but with relatively heavy systems (30-40 kg) that use field shaping ferromagnetic materials. In contrast, a system with frequency tracking and no ferromagnetic materials was used in [11], where an estimated ηdc-load=70% was calculated. Here, no clear description of the driver's efficiency is given, as it is based on a COTS 50Ω system with added TX and RX loops. Emphasis was again given to the control of the link and transfer efficiency, rather than the dc-to-load efficiency. Other interesting attempts to increase the end-to-end efficiency have been presented in [12], [13], where ηee>60% have been achieved at close proximity.
Improved systems will provide a high frequency, cost effective and efficient solution for mid-range IPT in the absence of field-shaping techniques, allowing a light-weight system to be achieved. A system with a TX-RX coil size difference represents a more realistic system, where the receiver size is usually constrained by its application. This system should be able to achieve high efficiency for lower coupling factors, due to the smaller RX coil size. Furthermore, this system should be able to achieve high efficiencies even under situations where perfect alignment is not always achievable (e.g. electric vehicle or wireless sensor charging).